This invention relates to improvements in scanning electron microscopes and the like apparatus and more particularly to a scanning electron microscope and the like which can permit the observation of high-resolution secondary electron images of specimens over a wide operation range covering operation at low acceleration voltage and operation at high acceleration voltage.
An electron beam apparatus in which a primary electron beam is irradiated or projected on a specimen to be observed and secondary electrons released from the specimen are detected to produce a second electron image of the specimen is well known as, for example, scanning electron microscope. In the electron beam apparatus, a secondary electron detector is applied with a high voltage of about 10 kV to attract secondary electrons released from the specimen for the purpose of efficiently collecting and detecting the secondary electrons.
An electric field created by the high voltage applied to the secondary electron detector naturally permeates into the primary electron beam path, raising a problem that the primary electron beam undergoes unwanted effects of deflection due to the permeant field. When acceleration operation is carried out at a high acceleration voltage for the primary electron beam which is several kilovolts or more, the amount of deflection of the primary electron beam caused by the permeant field is negligibly small and no serious problem occurs but conversely, for acceleration operation at a low primary electron beam acceleration voltage which is several kilovolts or less, the deflection amount becomes large and can not be neglected.
Especially where the specimen to be observed is disposed in an interior space of an objective lens and the secondary electron detector is disposed above the objective lens, the unwanted deflection of primary electron beam due to the permeant field greatly degrades resolution of produced secondary electron images. This is because the primary electron beam subject to the unwanted deflection is greatly affected by off-axial aberration of the objective lens.
A countermeasure for solving the above problems has hitherto been proposed as disclosed in, for example, JP-A-62-186451, according to which a plurality of secondary electron detectors (applied with high voltage) are provided which are differently distant from the optical axis of the primary electron beam and either one of the plurality of detectors is selectively used in compliance with the high or low level of the acceleration voltage for the primary electron beam (see FIG. 1).
Disadvantageously, the prior art apparatus must include the plurality of secondary electron detectors and accommodate a sufficient space for installation of the plurality of secondary electron detectors and in addition, because of the plural (2 to 4 at the most) detectors switched over for use, it is difficult to always maintain the optimum resolution by continuously complying with the change of acceleration over the wide range covering operation at low acceleration voltage and operation at high acceleration voltage.